Metabolic storage disease like mucopolysaccharidosis can potentially be cured by ex vivo transfer of the deficient gene into autologous hematopoietic stem cells followed by infusion of the genetically corrected cells. Transduction efficiency into hematopoietic stem cells of large animals and humans though has been too low to achieve a therapeutic effect. In this proposal, we wish to evaluate strategies with low toxicity to increase engraftment of hematopoietic stem cells transduced with Moloney murine leukemia virus (MoMLV) vectors transduced marrow cells in the absence of conditioning or infusions after partial marrow ablation with either hydroxyurea, methotrexate or busulfan are at least as effective as 200 cGy TBI, shown to be more effective in our past studies. Another hurdle towards gene therapy is the possible induction of immune responses to transduced cells and the transgene encoded proteins. We propose to characterize the immune response in normal beagles to autologous marrow cells transduced with the "foreign" human IDU gene to correlate the long-term persistence of these transduced cells with the host's immune response. We wish to develop methods to prevent or decrease this response by immunosuppressive treatment after transplantation. The most effective method will be tested in IDU-deficient dogs which will receive autologous marrow cells transduced with the "foreign" canine IDU gene followed by immunosuppressive treatment. Hematopoietic stem cells are considered to be predominantly quiescent. Gene transfer into hematopoietic stem cells by MoMLV vectors is not efficient in part due to their inability to transduce non-dividing cells. As human immunodeficiency virus type 1 (HIV-1) based vectors have been shown to infect non-dividing target cells, we wish to compare the efficiency of HIV-1 vectors containing the canine IDU gene with corresponding MoMLV vectors to transduce stem cells of normal and IDU deficient dogs. The outlined studies will address current problems of gene transfer and, if successful, will facilitate future gene therapy of Hurler syndrome and other diseases.